1. Field of the Invention
The present invention relates to a steel product for oil country tubular good, and more specifically, to a steel product for oil country tubular good having high SSC (sulfide stress corrosion cracking) resistance.
2. Description of the Related Art
Oil country tubular goods are used for collecting and producing crude oil or natural gas. The oil country tubular good has its both ends threaded, and as the drilling proceeds to deeper levels in an oil or gas well, a plurality of such goods are successively connected. At the time, the goods are subjected to stress by their own weights. Therefore, the oil country tubular good must have high strength.
The oil country tubular good must have SSC resistance, because it is used in a moist (sour) environment containing hydrogen sulfide. Sulfide stress corrosion cracking is caused when stress acts upon the steel product in the sour environment, and the higher the strength of the steel product is, the lower the SSC resistance becomes. In an oil country tubular good with high strength in particular, cracking can easily be propagated. Therefore, in order to improve the SSC resistance of an oil country tubular good with high strength, the crack arrest toughness of the SSC must be improved.
The following measures for improving the SSC resistance of an oil country tubular good with high strength have been reported.
(1) To temper steel at high temperature after quenching the steel. To add Cr, Mo, V, and the like to the steel in order to improve the hardenability and the resistance to temper softening.
(2) To refine the grain size of the steel (see Japanese Patent Laid-Open Nos. S61-223166 and H03-20443).
(3) To prevent prior austenite grain boundary cracking (see Japanese Patent Laid-Open No. H04-21718).
However, steel products subjected to these measures (1) to (3) were evaluated for their SSC resistance based on a tensile test or a bending test such as Method A test or Method B test defined by NACE TM0177. A smooth specimen is used for these tests, and therefore the tests do not evaluate the crack arrest toughness of the SSC. Therefore, a steel product evaluated to have high SSC resistance may suffer from SSC when potential cracking in the steel propagates.
In recent years, as wells have come to be drilled deeper, even higher SSC resistance is requested for oil country tubular goods. Therefore, in order to further improve the SSC resistance, it is preferable to prevent not only SSC initiation but also SSC propagation.
According to the disclosure of Japanese Patent Laid-Open No. H06-116635, the steel having a high Ni content can reduce SSC caused by propagation of potential cracking. However, Ni is expensive and the use of Ni increases the manufacturing cost of the steel product.